Mixed bed systems containing anion and cation exchange resins for the purification of aqueous solutions have many industrial applications. One application of such a system is in the purification of water for condensate recirculation systems used to drive steam turbines. It is essential that the water be of an extremely high purity level to avoid any adverse effects on the surfaces of blades, boilers and pipes of the turbine. This high purity level is particularly important for secondary condensate treatment in pressurized water reactor (PWR) systems of nuclear power plants.
Since it is desired to produce water that is free of any residue upon evaporation, the cation exchange resin should be in the hydrogen or ammonium form, and the anion exchange resin should be in the hydroxide form. In any event, it is conventional (as described in U.S. Pat. No. 3,582,504 which issued on June 1, 1971 to the assignee of this application and which is incorporated herein by reference) to regenerate a cation exchange resin with a strong acid such as sulfuric or hydrochloric acid, and to regenerate an anion exchange resin with a strong base, generally sodium hydroxide.
A particular problem with mixed bed ion exchange systems of the type conventionally employed is the production of ion "leakage," such as sodium ion leakage from a cation exchange resin or chloride ion leakage from an anion exchange resin. The term "leakage" refers to any ions that are not removed from the water by the ion exchange resin and, thus are permitted to "leak" past the resin. As used herein, the term "leakage" also refers to any undesired ions, such as sodium, which are introduced into the water by the resin itself.
The leakage problem arises primarily from the difficulty of obtaining perfect separation of anion and cation resins in a mixed bed before regeneration of the resins or from incomplete removal of residual undesired ions during a previous regeneration step. As used herein, the term "separation" refers to the bulk classification of resins within a single vessel or zone. The term "isolation" refers to the transfer of resins so that each resin occupies a separate zone.
As is familiar to those skilled in the art, separation is usually accomplished by passing water upwardly through the resins. This stream of water stratifies the resins by carrying the less dense anion exchange resin to the top of a separation vessel, while the more dense cation exchange resin is allowed to sink to the bottom of the vessel.
While the above method is effective for separating the major portion of the resins, perfect separation cannot be achieved. A primary source of this difficulty is the resin fines produced during handling of the resins. Since upflow separation depends upon particle size along with density, the cation exchange resin fines will not sink to the bottom of the separation vessel, but will be carried upwardly with the anion exchange resin. When the resins are subsequently isolated from one another, and the anion exchange resin is regenerated with sodium hydroxide, sodium ions will be introduced into the ion exchange sites of the cation resin contaminant. Thus, when the resins are returned to the service column, sodium ions will be introduced into the water being treated, producing sodium leakage.
This leakage problem is aggravated when ammonium hydroxide is introduced into the stream of the aqueous solution being treated, as is often done in condensate polishing systems to prevent corrosion. The ammonium ions promote the release of excess sodium ions from the cation exchange resin by gradually converting the resin to the ammonium form.
Moreover, ammonia and other volatile amines can be added in trace amounts to the steam of a high pressure boiler or turbine system to help minimize corrosion. The amines often appear in the condensate, however, and reduce the capacity of the resins to remove corrosion products and traces of electrolytes produced by leakage from the condenser.
A separate bed of a strongly acidic cation exchange resin can be placed in line and in front of the mixed bed of anion and cation exchange resins to pretreat the condensate and to remove the ammonia and volatile amines before passage through the mixed bed thereby reserving the capacity of the mixed bed resins to remove leakage and corrosion products. The use of a separate or single bed of strongly acidic cation exchange resin, however, requires the use of separate service vessels and regeneration equipment along with a large excess of fresh or unused acid to regenerate the resin.
The present invention overcomes these disadvantages through the use of a bed of filter materials including a first layer of a weakly acidic cation exchange resin as an active interface that removes impurities including metal ions and ammonia from a solution. A second layer includes a mixture of strongly acidic cation exchange resin and an anion exchange resin. That assembly, as described herein, is a tri-media or three component resin system for treating condensate.
Such a treatment system permits the use of a mixture of a strongly acidic cation exchange resin and a strongly basic anion exchange resin in the second layer with substantially no interference in the operation of the bed from ammonia and volatile amines present in the solution being treated. In effect, the weakly acidic cation exchange resin of the first layer functions to pretreat the solution before the solution is passed through the second layer. Moreover, the process occurs in a single vessel which provides a substantial cost benefit.
The weakly acidic anion exchange resin in the first layer also enhances the operating efficiency of the system by extending the run length of the mixed bed or mixture of the second layer. Runs can be terminated when the ammonia "breaks", that is, when the ammonia concentration in the aqueous solution or condensate exhausts the acidic cation exchange sites of the weakly acidic cation exchange resin. This is also referred to as the "ammonia breakthrough point". In preferred practice, however, the run can be terminated after the ammonia breakthrough point of the weakly acidic cation exchange resin of the first layer is reached, but before the ammonia breakthrough point of the second layer is reached.
As disclosed herein in the operation of the system the ammonia breakthrough point occurs before the sodium breakthrough point. Thus, the weakly acidic cation exchange resin of the first layer will continue to remove sodium and other metal ions from a solution even after the ammonia breakthrough point for that resin has been reached. Moreover, in certain applications it may be desirable to continue to use the tri-media bed even after the ammonia breakthrough points of the resins in the first and second layers have been reached. In such case, ammonia would pass through both layers of the bed and only metal ions would be removed from the solution.
In addition, in the absence of significant amounts of dissolved salts or air, the three component or tri-media resin bed of the system can be removed from service, physically cleaned and the weakly and strongly acidic cation exchange resins individually regenerated with a relatively low quantity of a strong acid. Further, ammonia can be stripped from the cation exchange resins and mixed bed polishing can be effected in a single vessel thereby providing a substantial cost benefit.
Another benefit of the present method is that a tri-media system provides a service bed includes only active components. Thus, the need for an inert media to act as a buffer zone to separate the strongly acidic cation exchange resin and the strongly basic anion exchange resin is eliminated. Surprisingly, the weakly acidic cation exchange resins described herein increase the efficiency of the bed for removing contaminants and facilitate the isolation of the strongly basic anion and strongly acidic cation exchange resins by providing an active interface resin buffer between those resins after the three resins of the system are separated. As a result, contamination of the strongly acidic cation exchange resin by the strongly basic anion exchange resin, and vice versa, is substantially reduced.
In addition, although weakly acidic cation exchange resins have been previously used for mixed bed operations, such resins have only been used in processes which are run at low flow rates with solutions having relatively high concentrations of solids; for example, in the production of liquid sugar. Weakly acidic cation exchange resins have not been used in the treatment of condensate, particularly in processes that are run at high flow rates and low solids concentrations.